Attribution is the use of analytic models to assign credit for customer responses (typically sales) to exposure of marketing from various media channels. The ultimate goal of such an analysis is to more effectively or efficiently allocate marketing expenditures by understanding each channel’s individual contribution and synergy with other channels.
One of the most common attribution approaches assigns total credit by order of media exposure (either first media or last media being credited). Other approaches apply more sophisticated schemas to assign credit across all media exposures. In this case, credit can be estimated by regression of aggregated exposure levels to aggregated actions, by application of user-level conversion information, or by a combination of both. 
Attribution Theory is a field of psychology which deals with the process by which individuals arrive at explanations for events. Typically, it includes an examination of the quantity and type of information gathered and how this information is combined to form a causal judgment. This theory is often applied to understanding brand preference and purchase behavior. 
- Common Language in Marketing Project Team, 2018.